Sand-teflon means to improve low current interruption performance of high voltage current limiting type fuses



Nov. 22, 1966 R. F. HUBER ETAL 3,287,524

SAND-TEFLON MEANS TO IMPROVE LOW CURRENT INTERRUPTION PERFORMANCE OF HIGH VOLTAGE CURRENT LIMITING TYPE FUSES Filed Oct. 28, 1964 M7 75 ,fi *5 IIIIIIIIIIII: 'IIIIIIIIIII! 'II'IIIIIIIA United States Patent SAND-TEFLON MEANS TO IMPROVE LOW CUR- R E N T INTERRUPTION PERFORMANCE OF HIGH VOLTAGE CURRENT LIMITING TYPE FUSES Ronald F. Huber, Evanston, and 'Ramanlal R. Vadnagara, Chicago, Ill., assignors to Joslyn and Supply Co., Chicago, Ill., a corporation of Illinois Filed Oct. 28, 1964, Ser. No. 407,051 1 Claim. (Cl. 200-120) This invention relates to high voltage fuses, and more particularly, to current limiting-silent fuses of the silversand type.

The silver-sand fuse is commercially used in high-voltage fuses of the current limiting-silent type to interrupt major faults or short circuit currents. However, the elementary silver-sand arrangement does not perform satisfactorily on interruption of low fault currents of long duration, such as overloads. The poor performance on low fault currents is due in part to the relatively slow growth of the arc length and insufficient dielectric recovery of the heated sand.

Because of the unsatisfactory low current interruptlon performance of the elementary silver-sand arrangement, a number of solutions have been proposed in the past. U.S. Patent 2,143,038, granted January 10, 1939 to Smith uses boric acid in part of the fuse as the low current interrupting means. U.S. Patent 2,294,767, granted September 1, 1942 to Williams uses a mechanical means of enlarging a gap to assist interruption. U.S. Patent 2,808,487, granted October 1, 1957 to Jacobs, uses a silver fusible element in ribbon form, perforated or otherwise, with a glass cloth chamber over a part of the fusible element. U.S. Patent 2,833,891, granted May 8, 1958 to Jacobs uses a silver fusible element in ribbon form with special perforations. All these arrangements are complicated and expensive to manufacture and are limited as to the geometry of the fusible element and are likely to have high interruption energies and long arcing times.

Accordingly, one object of the present invention is to provide a high voltage fuse which overcomes the above mentioned difliculties.

Another object of the present invention is to provide a new and improved high voltage fuse which will interrupt reliably at low fault loads.

Another object of the present invention is to provide a new and improved high voltage fuse which is inexpensive to manufacture.

Yet a further object of the present invention is to provide a new and improved high voltage fuse.

Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of the specification.

In accordance with these and many other objects of the present invention, there is provided a new and improved high voltage current-lirniting fuse, wherein a sleeve of insulating and arc-quenching gas-liberating material forms a chamber along a length of the fuse link. The fuse link, as is well known, is then imbedded in a casing filled with pulverulent inorganic arc-quenching filler. The new fuse utilizes the thermal insulation characteristic and/ or are extinguishing characteristic of polytetrafluoroethylene, commonly known under the trade name of Teflon, or similar material to provide simple and economical means to improve the low current interruption performance of high voltage current limiting type fuses.

The improved fuse is adaptable to any cross-sectional geometry, wire, ribbon or tubular, of the fusible element in any essential disposition. of the fusible element in suitable pulverulent interrupting medium, .for example, sand.

3,287,524 Patented Nov. 22, 1966 The fusible element may be of silver or other suitable material such as copper. However, silver is the preferred material from other considerations. The method, according to the present invention, can be applied to other types of fuses as well to improve the low current interruption performance.

For a better understanding of the present invention reference may be had to the accompanying drawings wherein:

FIGURE 1 is a longitudinal broken-away view taken through an improved high voltage fuse according to the present invention;

FIGURE 2 is an enlarged partial view of the fuse link and sleeve assembly according to the present invention;

FIGURES 3, 4 and 5 are also partial views of fuse link and sleeve assemblies according to additional embodiment of the present invention;

FIGURE 6 is a cross-sectional view of the fuse link and sleeve assembly of FIGURE 5, taken along line 66 of FIGURE 5;

FIGURE 7 is a partial view of a fuse link and sleeve assembly according to another embodiment of the present invention;

FIGURE 8 is a cross-sectional view of the fuse link and sleeve assembly of FIGURE 7, taken along line 88 of FIGURE 7; and

FIGURE 9 is a partial view of fuse link and sleeve assemblies according to yet another embodiment of the present invention.

Referring to the drawings, and particularly to the embodiment of FIGURES 1 and 2, there is illustrated an improved current-limiting fuse 10. The fuse 10 includes a tubular casing 11 of insulating material forming a chamber, the respective ends of which are closed by a pair of electrical terminals 12 and 13. A fuse link 14 conductively interconnects the terminals 12 and 13. As is know in the high voltage fuse art, the fuse link 14 may have any cross-sectional geometry, but is here shown as circular in cross section, and is formed of suitable fuse material of relatively low resistivity and with a relatively low specific heat such as silver. The fuse link 14 may, if desired, have incorporated any M-effect, low melting point alloy soldered to the fusible element to form a body 15 of metal. As is well known, the body 15 of low fusing point metal will destroy the fuse link 14 by a metallurgical reaction taking place at a temperature below the fusing point of the fuse link 14 thereby to provide for rupture of the fuse link 14 upon inadmissable duration of low overloads. The body 15 of M-etfect metal may suitably be formed of tin or tin alloys, or indium which will form alloys with the metal of the fuse link 14.

In accordance with the present invention, a sleeve 16 of Teflon or other suitable insulating and arc-quenching, gas-liberating material forms a chamber 16a along the fuse link 14 symmetrically about a point in the fuse link, here illustrated as the body 15 of M-etfect material- The Teflon sleeve 16 will, of course, have a cross-sectional shape to accommodate the fuse link 14, and in the illustrated embodiment is round or tubular. Means are provide at the ends of the chamber to allow the escape of gaseous material from the chamber to the interrupting medium and to prevent the pentration of interspace, the space between the inside of the chamber and the outer surface of the fusible element, by the particles of the interrupting medium. The fuse link and sleeve assembly is surrounded within the casing 11 by a suitable pulverulent interrupting medium having arc quenching properties, such as sand 17.

Referring now to the embodiment of FIGURE 3, there is illustrated a fuse link and sleeve assembly 20 including a fuse link 21 having a Teflon sleeve 22 forming a cham ber 2201 about a section of the fuse link 21. As illustrated in FIGURE 3, M-effect material 24 is incorporated in the fuse link 21 and the Teflon sleeve 22 is positioned symmetrically with reference to M-efiect material 24. In the illustrated embodiment, a glass cloth sleeve 25 is slipped on the Teflon chamber just for added thermal insulation.

Referring now to the embodiment of FIGURE 4, there is illustrated a fuse link and sleeve assembly 30 including a fuse link 31 incorporating M-effect material 32. A sleeve 33 of Teflon or other suitable material is symmetrically positioned along the fuse link 31 symmetrically with reference to the M-effect material 32 to provide a chamber 33a. End enclosures 34 are provided to prevent the penetration of the interspace by the pulverulent material surrounding the fuse link assembly 30. If desired, a suitable glass cloth sleeve 26 may be provided on the Teflon sleeve 33.

FIGURE and 6 illustrate a fuse link formed of parallel ribbons of fusible elements. As therein illustrated there is shown a fuse link and sleeve assembly 50, wherein the fusible elements consist of two ribbons 51, 52 of fusible material in parallel. A Teflon strip 53 is sandwiched between the fusible element ribbons 51 and 52. Spaced Teflon end sections 54, 55 are interconnected by a central tubular section 56 of Teflon forming a heat insulating and arc-quenching chamber 56a. In the illustrated embodiment, suitable M-effect material 57 is incorporated in the respective ribbons 51 and 52 of the fuse link.

FIGURES 7 and 8 illustrate the invention applied to a tubular fusible element. Astherein illustrated, there is shown a fuse link and sleeve assembly 60, including a tubular fusible link 61. Throughout a selected portion of the-fusible element 61 there is provided an inner core of Teflon 62, here illustrated as in tubular form, but which also may be in rod form. The hollow center of the Teflon inner core 61 and the tubular fusible element 61 are filled with suitable inorganic filler 63, for example, of glass fiber.' Concentrically positioned over the inner core 62 are spaced Teflon end sections 64, 65 interconnected by a central section 66 of Teflon forming a chamber 66a. Suitable M-eflect material 67 may be incorporated in the fuse link 61.

Referring now to the embodiment of FIGURE 9, there is illustrated a fuse link and sleeve assembly 70 including a fuse link 71 incorporating M-eflect material 72. A pair of spaced sleeves 73 and 74 0f Teflon or other suitable material is positioned along the fuse link 71 symmetrically with reference to the M-effect material 72. The thermal insulating and arc extinguishing characteristics of the Teflon sleeves are effective to improve the low current interruption performance of the fuse; however, the effects of the sleeves are not as pronounced as in the previously described embodiments having'a chamber along the fuse link.

Referring to FIGURES 2 through 8, it will beseen that the chamber formed by the Teflon or Teflon and glass sleeve is effective to initiate the melting of the fusible element at about the midpoint of the portion occupied by the chamber at desired low over currents within desired times. For a given fusible element, with or without M- effect, andinterrupting medium, this interruption characteristi-c can be controlled by the location of the chamber along the fusible element and the construction and dimensions of the chamber as illustrated in FIGURES 2 through 8. The chamber is also effective to quench the are upon melting off the fusible element within the chamber and limiting thearcing time and are energy under low current interruption.

Referring to FIGURE 9, it will be seen that the melting of the fusible'element is initiated by M-eifect or other means at a point between two Teflon sleeves. The low current are is then interrupted by the Teflon. This particular arrangement then ensures, if desired, higher insulation after interruption. The are quenching action of the Teflon is believed to be due to the electronegative character of fluorocarbon gases, such as CF which use the main components of decomposition of the Teflon.

Tests on high voltage fuses constructed in accordance with the present invention indicate that the interruption energies involved are small in the low current range when the melting of the fusible element is confined only to the portion occupied by the chamber formed by the Teflon sleeve. The interruption energies could be reduced to relatively minute amounts by a suitable form of chamber. The arcing time may be reduced to maximums of one-half cycles. The high current interruption performance of the fuse is unaffected by the presence of the Teflon portions in the chamber. Accordingly, the proposed improvement renders the interrupting ability of the silversand fuse satisfactory over the entire range of faultcurrents.

Although the present invention has been described by reference to several embodiments thereof, it will be apparent that numerous other modifications land embodiments will be devised by those skilled in the art which will fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

A high voltage current limiting fuse comprising a casing of insulating material, a pair of terminal elements closing the ends of said casing, a uniform fuse link of round wire free of M-elfect material conductively interconnecting said terminal elements, sleeve means of Teflon spaced around a portion of said fuse link, sleeve means of glass cloth provided over the above mentioned sleeve means, end enclosure means closing the ends of said Teflon sleeve, and a pulverulent arc-quenching filler surrounding said fuse link and said sleeve means within said casing.

References Cited by the Examiner UNITED STATES PATENTS 1,829,149 10/1931 Mason et al. 200143 2,502,992 4/ 1950 Rawlins et al 200-12O X 2,531,007 11/1950 Strorn et al. 200149 2,665,348 1/1954 Kozacka 200 2,808,487 10/1957 Jacobs 200120 2,833,890 5/1958 Jacobs 200120 2,856,488 10/ 1958 Kozacka 200120 BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner. 

